1. Field of the Invention
The present invention relates to a motor drive circuit, a fan motor, electronic equipment, and a notebook computer.
2. Description of the Related Art
In electronic equipment such as a notebook computer, a fan motor is used for cooling a heat-producing component such as a processor. In the fan motor, a drive voltage applied to a motor coil is controlled according to a signal indicating a rotational position of the motor, so that the motor runs at the desired rotation. In a case where the drive voltage applied to the motor coil is thus controlled, change of the drive voltage may be moderated for silencing of noise, reduction of flyback voltage, etc. (see Japanese Publication Laid-Open No. 2004-166379.)
When using the fan motor to cool a component, a cooling effect can be enhanced by increasing rotational speed of the motor, i.e., rotational speed of a fan. However, constantly keeping the fan at high rotation speed causes problems such as an increase in power consumption, fan noise. Therefore, the rotational speed of the fan is often controlled to be at a necessary level by intermittently driving the motor according to an amount of heat released by a component to be cooled, an amount of processing, etc.
FIG. 6 is a diagram showing an example of a configuration of a motor drive circuit that drives a single phase fan motor. In a motor drive circuit 100, drive voltages VOUT1 and VOUT2, which are applied to terminals OUT1 and OUT2 connected to a motor coil L, are controlled based on voltages VH1 and VH2 opposite in phase from each other according to the rotational position of the motor that are output by a Hall element 110. The voltage of a connection point between a P-channel MOSFET 112 and an N-channel MOSFET 113 connected in series is denoted as the drive voltage VOUT1. An operational amplifier 115 controls a voltage applied to gates of the P-channel MOSFET 112 and the N-channel MOSFET 113 so that the drive voltage VOUT1 becomes a voltage according to a difference between the voltages VH1 and VH2 output by the Hall element 110. The drive voltage VOUT2 is controlled to have a phase opposite to that of the drive voltage VOUT1 based on the voltages VH1 and VH2 output by the Hall element 110. Accordingly, in the motor drive circuit 100, the drive voltages VOUT1 and VOUT2 are controlled to change moderately, so that silencing of noise, reduction of flyback voltage, etc., are realized.
The motor coil L is intermittently controlled according to a PWM signal output by a PWM signal output circuit 120. Specifically, when the PWM signal is at H level, the P-channel MOSFETs 122 and 123 are OFF, and therefore, the drive voltage VOUT1 is controlled by the operational amplifier 115, however, when the PWM signal is at L level, the P-channel MOSFETs 122 and 123 are ON, and therefore, the P-channel MOSFET 112 is OFF, the N-channel MOSFET 113 is ON, and the drive voltage VOUT1 changes to the L level regardless of control by the operational amplifier 115. Similarly, the drive voltage VOUT2 is also controlled according to the PWM signal. Accordingly, the motor coil L is in a state of not being driven while the PWM signal is at L level. In other words, in the motor drive circuit 100, the rotational speed of the fan can be controlled by changing duty of the PWM signal according to circumstances.
FIG. 7 is a diagram showing an example of change of the drive voltage VOUT1 according to the PWM signal. When the PWM signal changes from the H level to the L level, the P-channel MOSFET 112 is turned OFF, the N-channel MOSFET 113 is turned ON, and the drive voltage VOUT1 quickly changes to the L level. On the other hand, when the PWM signal changes from the L level to the H level, the drive voltage VOUT1 changes under a feedback control by the operational amplifier 115, and therefore, time is required according to frequency characteristics of the operational amplifier 115 to reach a target level according to the difference between the voltages VH1 and VH2 output by the Hall element 110.
In a fan motor that cools a processor or the like of a notebook computer, the rotational speed of the fan is changed according to circumstances, and in such circumstances that little heat is released such as a standby state, sleep mode, etc., in order to reduce the power consumption, it is desirable to operate the fan at a low rotation speed as much as possible. Therefore, in a case of the motor drive circuit 100, in order to operate the fan at the low rotation speed, it is necessary to reduce a duty ratio of the H level of the PWM signal, i.e., a pulse width of the PWM signal.
However, in the motor drive circuit 100 as illustrated in FIG. 7, time is required according to the frequency characteristics of the operational amplifier 115 for the drive voltage VOUT1 to reach the target after the PWM signal changes from the L level to the H level. Therefore, as the pulse width of the PWM signal is reduced, the PWM signal may change to the L level and the drive voltage VOUT1 may undesirably change to the L level before reaching the target level. Accordingly, the rotational speed of the fan cannot be linearly controlled according to the duty of the PWM signal especially in a range of the low rotation speed, and therefore, it is difficult to operate the fan at a sufficiently required low rotation speed.